This invention relates to a film supplying apparatus and a lift mechanism for a packaging machine of the type adapted to use a polyethylene film to wrap up not only articles like fresh foods but also the tray carrying them thereupon.
In a packaging machine of this type, the film is usually pulled out of a roll, and both side edges of a film to be cut to a specified size are clamped between upper and lower film transporting belts and transported to a packaging station where it is kept stretched. Articles carried on a tray to be packaged together therewith are pushed upwards from below by a lift against this stretched film piece. While the loaded tray is in this lifted-up position, a plurality of folding plates are activated to fold the four edge parts of the film piece downward towards the bottom surface of the tray to complete the packaging process.
With a packaging machine of this type, films with different widths are used, depending on the size of the articles to be packaged, or the size of the tray which carries them and is to be packaged together with the articles carried thereon. In such a situation, not only must the roll having a film with an incorrect width therearound be removed and another roll having a film with a correct width installed in its place, but both side edges of the front end of the newly installed film must also be set so as to be clamped by the film transporting belts. In other words, it is extremely troublesome and time-consuming to keep using films with different widths.
In view of the problem as described above, it has been known to provide two film storage spaces on both sides of the film transport belts, each film storage space storing a film roll having a film with a different width such that either of them can be selectively used for trays of a given size. Even if a packaging machine is thus provided with a film supplying apparatus having two film storage spaces, however, care must still be taken such that the end of the film not being used will not be caught by the film transporting belts. Moreover, the film from the newly installed roll must be carefully pulled out from the roll and be set correctly to be clamped between the film transporting belts. Especially if a film must be manually moved into a storage space or set between the film transporting belts, it becomes wrinkled easily if the tension thereon is reduced and its handling becomes even more difficult. In summary, prior art film supplying apparatus are difficult to use and their workability is disappointingly poor.
It is therefore an object of this invention to provide a film supplying apparatus for a packaging machine which allows films to be interchanged automatically such that the general workability of the machine can be improved at the time of exchanging films.
In anticipation for occurrences of erroneous film movement, packaging machines of this type are generally provided with many sensors. Before a series of packaging operations is started, the wrinkled front edge part of the film is cut off, but the operator may forget to remove the edge part which has been cut off. Even without any negligence on the part of the operator, the stretched film piece by itself may miss and fail to catch the tray which is lifted up by the lift. In such a situation, the film may be transported farther beyond the packaging station toward its downstream side, becoming wound around and caught below the end parts of the film transporting belts. Such an "overrun" situation of the film is detected by an overrun detection sensor adapted to detect the presence of the film where it normally should not be, such that the film transporting belts and other film supplying means can be stopped as soon as such an abnormal situation is detected. It has also been known to provide a film detector on the upstream side of the film transporting belts for stopping the belts and other film supplying and driving means as soon as it is detected that the film roll has run out of the film or that the film supply to the film transporting belts has been interrupted, for example, because the film has been caught around a supply roller.
If two film storage spaces are provided on both sides of the film transporting belts, as explained above, and if the direction of motion of the film transporting belts is made switchable, however, it becomes necessary to provide overrun detection sensors and film detection sensors of the kinds described above at both ends of the belts. In other words, the total number of detectors will have to be doubled. This affects not only the cost of the parts but also the labor required to install these extra sensors.
It is therefore another object of this invention to provide a film supplying apparatus for a packaging machine which uses each of its sensors both as an overrun detection sensor and as a film detection sensor, depending on the direction in which the film is supplied to the packaging station, such that the total number of sensors does not have to be increased although the film transporting belts may be movable in two opposite directions.
The lift for lifting trays of different sizes usually comprises a vertically movable base member provided with a plurality of posts adapted to hold a tray thereon. Since trays come in different sizes, however, it is desirable to be able to vary the separations among the posts according to the size of the tray to be lifted. For example, their separations should be increased for a larger tray so that it can be carried stably, and they should be brought closer together in the case of a smaller tray because a narrower film will be used for wrapping a smaller tray and the film transporting belts on both sides are brought closer together, making it necessary to prevent collisions between the film transporting belts and the peripheral posts.
In order to make the separations changeable among the posts, it has been known to attach the posts slidably onto the base member. From the point of view of workability, it has also been known to provide sliders by means of which the posts can be moved horizontally and their density per area can be controlled, but these sliders must be able to move the posts in all and mutually opposite directions. This results in a different problem of alignment and engagement between the sliders and the posts. After a periodic cleaning of the apparatus as a part of its maintenance work or occurrence of an abnormal situation, furthermore, the movable posts are probably found at random positions. It is then a problem to be considered how efficiently one can rearrange randomly positioned posts by bringing them to specified positions.
It is therefore still another object of this invention to provide a lift mechanism for a packaging machine capable of easily and efficiently move its posts horizontally.